Submerged turbine aerator

ABSTRACT

A submerged aerator rotor for use with sparged air for water and waste treatment systems. The air is supplied evenly distributed azimuthally to the bottom of a circular shroud which is rotated by a vertical shaft from the surface of a treatment tank. The upper surface of the shroud is vaned with a clearance between the vane tip and the shroud edge.

[ SUBMERGED TURBINE AERATOR [75] Inventors: Francis J. Sisk, Pittsburgh,Pa.;

Chesley F. Garland, Tucson, Ariz.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: June 15, 1971 [21] Appl. No.: 153,336

[52] 11.8. C1. 261/93 [51] Int. Cl B011 03/04 [58] Field of Search.261/87, 93; 209/169, 170, 168

11] 3,792,840 1451 Feb. 19,1974

Primary ExaminerFrank W. Lutter Assistant Examiner-Vincent GiffordAttorney, Agent, or Firm-.1. R. Campbell [57] ABSTRACT References CitedA submerged aerator rotor for use with sparged air for UNITED STATESPATENTS water and wastetreatment systems. The air is supplied '2 521 3969 1950 M0111 2 61/93 evenly distributed azimuthany the b01101 of2,750,323 6/1956 Sampson et a1 261/93 X cular shroud which is rotated bya vertical shaft from 3,154,601 10/1964 Kalinske et a1. 261/93 he rf cef a tre ment nk. The upper urface of 3,256,987 6/1966 lsenhardt et aL.261/93 X the shroud is vaned with a c1earance between the vane 1,268,6306/1918 Ruth, Jr. 261/87 tip and the shroud edge. I 1,998,694 4/1935Weinigi... 261/87 2,243,302 5/1941 Weinig. 261/93 1 Claim, 4 DrawingFigures 1 4 P t F C h-,1; I 9 ,0.T. r 0 "ofo I ;:L}; Z;'2, j 1 f I Q Z JJ BACKGROUND OF THE INVENTION This invention pertains to processes andapparatus for enhancing the mass transfer of oxygen from a sparged airsystem into a liquid medium, and more particularly to submerged turbinerotors for accomplishing same. I

In water and waste treatment systems as for example the activated sludgesewage treatment process, one popular method of supplying oxygen for theaerobic metabolism of micro-organisms involves sparging air into aliquid below the plane of a submerged vaned rotor driven by a verticalshaft from above the liquid level. The geometry is generally such thatthe air passes through the vanes themselves.

It has been found that contrary to the usual view of such two-phase flowmechanics, the air is not dispersed in vane passage and entrained inliquid medium, but is rather separated into a film flow on the trailingface of the vanes and dispersed in a jet mixing flow exiting from therotor tip. The air consequently encounters only a small fraction of theradially outward streaming liquid. The air insufflated liquid in thearea of the rotor coalesces into a powerful central upwelling flowdriven by percolation head and the air vents through the surface in acentral boil while the accompanying liquid flows radially outward andsinks near the tank rim. In consequence, the path of the air to thesurface is short and the contact time of the air with the liquid mediumis less than might otherwise be the case. The mass transfer area anddriving force are thus minimized.

Moreover, the power draw of a rotor with large air ventilated cavitiesbehind the trailing faces of its vanes is much smaller than the powerdraw of the same rotor without such cavities since the presence of airconsiderably reduces the liquid flow and the vane drag coefficient.Thus, the drive must be specified for a considerably higher capacitythan the normal operating power to permit operation of the rotor duringan air supply casualty to prevent sludge buildup on the bottom of thetank.

SUMMARY OF THE INVENTION The contacting time and accordingly the masstransfer driving force is maximized in accordance with this invention bythe provision of a rotor with vanes only on the upper surface of therotor platform and having vane tips suitably displaced from the edge ofthe platform or shroud. v

Air is sparged from a ring onto the shroud at approximately theintersection of the shroud with a conical fairing projecting downwardlythereform. The conical fairing serves to preserve the uniform azimuthalair distr'ibution created by the sparge ring. The displacement of thevane tips from the shroud edge permits separation of the liquid vortexstream from the vanes before air flow is encountered and thus prohibitsair induction into an attached vortex and thus onto the vane trailingface. I

Air is accordingly delivered uniformly off the shroud underside into theunderside of the radial liquid jet which is driven by the vanes on theshroud upper side. The air is dispersed by the shear flow which itencounters and is driven radially outward while rising in discretebubbles through the liquid jet. By the time the air has freed itselffrom the jet entrainment and is capable of creating an upwelling flowdue to its percolation head, it has been radially dispersed. The resultis a ring of air insufflated liquid in upwelling flow at a large radialdistance from the rotor. At the surface the air disengages and theliquid separates into an inner toroidal vortex which returns to therotor eye and an outer torus which becomes a sinking flow at a largeradius or at the tank rim. The air contacting time with the water isthus maximized for a given rotor power.

DESCRIPTION OF THE DRAWINGS For a better understanding of the inventionreference may be had to the accompanying illustrative drawings, inwhich: 1

FIG. 1 shows a schematic cross-sectional elevation view of a tank havinga submerged turbine aerator in accordance with this invention;

FIG. 2 is a cross-sectional elevation view enlarged to show details ofthe turbine aerator;

FIG. 3 is a plan view of the turbine aerator viewed from above; and

- FIG. 4 is an expanded view of a vane of the aerator of FIG. 3 showinga means for supporting same.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, there isshown a liquid containment tank 10 suitable for the treatment of wateror waste water. A turbine aerator 12 is shown supported therein by acentral vertical shaft 14. The turbine aerator 12 is supported for usein conjunction with sparged air which is evenly distributed azimuthallyto same through a sparge ring 16 supplied with air through an inlet pipe18. v

In contrast with the prevalent design of turbine aerator systems, theturbine blades of this system are designed to operate in a single phaseliquid medium. Accordingly, the turbine blades 20 of the turbine aerator12 are mounted on the top surface of a flat circular shroud 22 and aredisplaced from the edge 24 thereof. More particularly, as can best beseen in FIG. 3, the tip 25 of each turbine blade 20 is displaced asubstantial distance from the edge 24 of the shroud 22. The shroud 22 isalso equipped with a conical lower fairing 26.

As previously indicated air is uniformly azimuthally distributed from asparge ring 16 onto the lower surface of the shroud 22 at approximatelythe intersection of the shroud 22 and the conical fairing 26. Theturbine aerator 12 is driven by a motor 30 and gear system 32 via avertical shaft 34 connected via coupling 36 to a suitable fitting 38affixed centrally of the shroud 22; as is well known to the art.

The-rotation of the turbine aerator 12 in the single phase liquid mediumcauses a radially outward liquid jet to form above and outwardly of theshroud 22. Air bubbles from the sparge ring 16 are delivered uniformlyfrom the shroud edge 24 into this jet stream and are driven radiallyoutward while rising in discrete bubbles through the liquid jet. By thetime the air has freed itself from the jet entrainment it has becomeradially dispersed. Accordingly, the path of any air bubble is muchlonger than it would have been had it passed through the turbine blades.This means that the air contacting time with the liquid medium has beengreatly increased resulting inan increase in the mass transfer rate ofoxygen to the water in the tank 10. FIG. 1 generally shows a typicalpath for the air bubbles and the .main liquid streams. As can be seen,the liquid separates into an internal toroidal vortex 19 which generallyreturns to the eye of the turbine rotor 12 and at outer torus 21 whichbecomes a sinking flow adjacent the tank rim.

The rotor vanes are shown radially oriented and supported by supportrods 39 welded both to the vanes 20 and the shroud 22; see.FlG. 4.Although the vanes 20 are shown along radii of the circular shroud 22,they may be angled with respect thereto with a probable increase inefficiency. What is essentially required is that the impeller or vaneportion of the aerator 12 work in a single phase liquid alone anddischarge that liquid with a large radial velocity component and thatthe gas be delivered to this shear flow substantially uniformly and at apoint sufficiently separated from the vanes 20 that the upwelling of thegas cannot cause the entrainment of the air bubbles on the suctionsideof the impeller vanes 20. This last desideratum is accomplishedeconomically as can best be seen in FIG. 2 by separating the vane tipsfrom the edge 24 of the shroud 22 by a distance which is a substantialportion of the height and pitch of the vanes 22. FIG. 2 shows thisdistance to be approximately 50 percent of both the height and pitch ofthe vanes. Routine experimentation with various rotational velocities ofthe turbine aerator 12 and varying gas flow rates will indicate theseparation distance required for any particular system. As is known,impeller blades generally have impeller tip vortices which'extend somedistance outwardly from the vane tips. An exception exists where thevanes are curved to closely fit the flow field. In this last mentionedcase, the bubbles may be fed into the flow field at the tips of thevanes. Otherwise, the bubbles must enter the field outwardly of thevortices.

What we claim as our invention: 1. An aerator adapted for submergedoperation comprising:

a rotatable shaft supporting a flat shroud of circular configurationadapted for submergence in a liquid; a drive train connected to theother end of said shaft; impeller vanes mounted on the upper surface ofsaid shroud; an inverted conical fairing attached to the underside ofsaid shroud, said shroud extending radially beyond the outer end of saidimpeller vanes; air sparge means mounted beneath said shroud andconcentrically disposed with respect to said conical fairing forazimuthally distributing air onto the lower surface of said shroud;whereby upon rotation of the impeller, a continuous liquid jet isdirected radially outward of the shroud, and as the air spills fromunder the shroud into the jet, it is sheared into small bubbles whichbecome dispersed in the liquid and provide maximum transfer of oxygenthereto.

1. An aerator adapted for submerged operation comprising: a rotatableshaft supporting a flat shroud of circular configuration adapted forsubmergence in a liquid; a drive train connected to the other end ofsaid shaft; impeller vanes mounted on the upper surface of said shroud;an inverted conical fairing attached to the underside of said shroud,said shroud extending radially beyond the outer end of said impellervanes; air sparge means mounted beneath said shroud and concentricallydisposed with respect to said conical fairing for azimuthallydistributing air onto the lower surface of said shroud; whereby uponrotation of the impeller, a continuous liquid jet is directed radiallyoutward of the shroud, and as the air spills from under the shroud intothe jet, it is sheared into small bubbles which become dispersed in theliquid and provide maximum transfer of oxygen thereto.